Augmented hippocampal up-regulation of immune modulators following a peripheral immune challenge in a hemizygous mouse model of the 15q13.3 microdeletion

Abstract

The strongest known genetic risk factor for generalized epilepsy is the human hemizygous 15q13.3 microdeletion (MD). This 1.5 Mb MD encompasses six genes, including CHRNA7 encoding the alpha7 subunit that forms the homo-pentameric nicotinic acetylcholine receptor, a known regulator of the immune system. In the CNS, hyper activation of neuroimmune responses contributes to increased seizure susceptibility. In a mouse model with a hemizygous deletion of the orthologous region (Df(h15q13)/+) (Het), we previously demonstrated increased hippocampal expression of inflammatory cytokines compared to wildtype (WT) mice following a mild peripheral immune challenge. To further characterize neuroimmune responses, hippocampal mRNA expression of the chemokines CXCL2 and CXCL10, and the Gap junction protein connexin 43 (GJA1), all of which are implicated in neuronal hyperexcitability, were determined along with additional immune related targets. Three hours after a lipopolysaccharide (LPS, 0.1 mg/kg) or polyinosinic:polycytidylic acid (Poly(I:C), 5 mg/kg) injection (i.p.), hippocampi were collected, mRNA extracted, and cDNA prepared for qPCR. The results demonstrate extensive upregulation of CXCL2 and CXCL10 expression by LPS and Poly(I:C) (up to 200-fold CXCL2, up to 600-fold CXCL10) (p < 0.0001) with genotype x treatment interactions for CXCL2 by LPS (p < 0.007). Responses to treatment were far smaller in magnitude for all other targets. LPS and Poly(I:C) induced statistically similar increases for Toll-like receptor (TLR)2, TLR4, HMGB1, and C3, but Poly(I:C) had stronger effects on GJA1, TLR3, C1qA and MARCO expression. Remarkably, TLR3 was the only target with significant downregulation of expression after Poly(I:C) (p < 0.0001). In addition, genotype x treatment interactions were detected for TLR3, TLR4, HMGB1, and C1qA (p < 0.02). Thus, a peripheral immune challenge caused extensive increases for CXCL2 and CXCL10, and the genotype-treatment interactions that was seen for several targets, underscored the augmented neuroinflammatory response in mice carrying the MD. Of note is the dramatic upregulation of CXCL10 by low dose Poly(I:C). CXCL10 causes hyperexcitability via neuronal CXCR3 activation. Thus, even an asymptomatic viral infection may increase seizure susceptibility. In summary, a peripheral immune challenge causes strong upregulation of hippocampal inflammatory mediators implicated in neuronal excitability which is particularly detrimental for individuals with high seizure susceptibility, such as carriers of the 15q13.3 MD.